Now that metallocene catalysts are being used for the commercial scale production of polymers, it has become increasingly important to optimize the polymerization process and polymer products. One key variable is the catalyst system itself Any alteration in the catalyst system formulation which results in increased efficiency, improved productivity, reduced cost, or improved product is extremely valuable. Reducing the amount of metallocene loaded on the support would be expected to result in a reduction in the number of catalyst active sites which would be expected to lead to lower catalyst productivity. We have unexpectedly found, however, that reduction of the metallocene loading not only reduces catalyst cost but actually increases catalyst productivity and improves polymer bulk density. Additionally we have found that using support materials having small particle size improves operability and productivity.
Generally prior art supported metallocene catalyst systems useful for the production of polyethylene use at least 0.06 mmol of metallocene for each gram of support material. The productivity, polymer particle size and bulk density of these catalyst systems is often too low for commercial processes.
WO 9 600 245 describes a method for forming silica supported catalyst systems. In Examples 1-2 Davison 948 silica is used which has an average particle size of from 50-60.mu.. The ratio of metallocene metal to silica used in the preparation of the catalyst system is 0.069 mmol/g. The catalyst system was tested in a continuous gas phase reactor in the polymerization of ethylene with hexene. The bulk density was 0.426 g/cc and the average particle size was 511.mu.. The productivity was 1351 g polymer/g catalyst.
WO 9 600 243 describes methods for supporting metallocene catalyst systems for the polymerization of propylene. In the examples, Davison 948 silica is used as are chiral bridged metallocenes useful for the polymerization of isotactic polypropylene. The amount of these highly active metallocenes applied to the support is very low, for example around 0.0187 mmoles Zr/g support. In the examples, propylene was polymerized in a batch slurry reactor. There is no indication of average particle size or bulk density.
It is among the objectives of this invention to provide supported catalyst systems useful for the gas and slurry polymerization of ethylene polymers on a commercial scale with good productivity, low fouling, high bulk density, and optimal particle size. We have surprisingly found that a reduced loading of metallocene on the porous support helps to provide this balance of properties.